imposm / osm-bright fails with UnicodeEncodeError

Using imposm fails with an UnicodeEncodeError when processing relations.

I processed an OSM file from Geofabrik (alps.osm.pbf). At the end it failed with the following error repeated 4 times, and the process hanged.

Last message was: relations: 10k/79k. TileMill complains about missing relations.

$ imposm -U postgres -d osm -m mapbox-osm-bright-c9bc5ee/imposm-mapping.py --read --write --optimize --deploy-production-tables alps_20130208.osm.pbf 
loading mapbox-osm-bright-c9bc5ee/imposm-mapping.py as mapping
password for postgres at localhost:
[11:38:55] ## reading alps_20130208.osm.pbf
[11:38:55] coords: 78954k nodes: 1579k ways: 11279k relations: 78k (estimated)
[11:51:19] coords: 103636k nodes: 365k ways: 12045k relations: 73k
[11:51:21] reading took 12m 25s
[11:51:21] ## dropping/creating tables
[11:51:25] ## writing data
Exception in thread Thread-1:
Traceback (most recent call last):
  File "/usr/lib/python2.7/threading.py", line 552, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.7/threading.py", line 505, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/usr/local/lib/python2.7/dist-packages/imposm/dbimporter.py", line 87, in db_importer
    db.insert(mapping, insert_data)
  File "/usr/local/lib/python2.7/dist-packages/imposm/db/postgis.py", line 104, in insert
    self.cur.executemany(insert_stmt, insert_data)
UnicodeEncodeError: 'ascii' codec can't encode character u'\xe0' in position 9: ordinal not in range(128)

Start with the traceback locations in imposm/dbimporter.py and imposm/db/postgis.py, then reproduce the failure with the shown imposm command and alps_20130208.osm.pbf input. Trace the relation-writing path and verify that processing completes without the UnicodeEncodeError or hang, with relations available to TileMill.